Try ensuring all desktop effects are off (via system settings) and maybe try adding firmware-linux-nonfree (from Debian sid non-free) to get the firmware for the video chip which just might improve things.

It should work better with the r300g driver than with software rendering. One thing I would recommend is using mesa 7.11 (from experimental) as there have been a lot of improvements in the r300g driver (and the gallium3d infrastructure in general) over 7.10.x.